This invention relates to heat exchangers, especially for motor vehicles. More particularly, the invention relates to metallic cooling fins used in heat exchangers of either the brazed type or the built-up type (in which the components are assembled together mechanically), the cooling fins defining indirect heat transfer surfaces which augment the heat transfer surfaces between the heat exchanger tubes (in which a first, hot or cold, fluid flows), and a second fluid, such as air, which flows around or between the tubes.
Such cooling fins are generally made in the form of plates which are superimposed one above another in a stack in a heat exchanger of the built-up type, and in that case the tubes extend through the stack of plates. In head exchangers of the brazed type, the cooling fins are generally accordion-shaped, i.e. they are corrugated, and in that case they are interposed as spacers or inserts between the tubes.
Some known types of cooling fins have a central part or middle section, which is formed with at least one set of fixed lamellae which are inclined (with respect for example to the axis of the tubes), and which are of selected form, being spaced apart by apertures of predetermined dimensions to enable the fluid to pass between the lamellae. This arrangement of lamellae is similar to that of a slatted window shutter, so that such fins can be referred to as slatted shutter fins: they are for example described in U.S. Pat. No. 5,289,874.
Although it is true that the provision of this slatted-shutter configuration on the indirect heat transfer surfaces of the cooling fins improves heat transfer performance, it does at the same time increase energy losses, and this reduces the output of the heat exchanger. This effect is increased, when a larger number of superimposed cooling fins is provided. It can be increased even more if the installation has several heat exchangers connected in series.
In addition, it is well known in the art that the slats in the slatted-shutter configuration, where these consist of lamellae of constant inclination, lead to energy losses which are greater as the inclination is greater, as is generally the case in known heat exchangers, in which the fins typically have an inclination of about 35xc2x0. Such angles give rise to separation of the boundary layer of the fluid flowing in contact with the cooling fins, at those locations at which the fluid, for example air, begins to change direction. As a result, low velocity zones, in which recirculation or cavitation of the air can occur, are created close to the walls. From the heat transfer point of view, such zones are detrimental because normal convection cannot take place in such zones.
An object of the invention is to provide a metallic cooling fin which reduces or eliminates the drawbacks, such as those mentioned above, of known types of cooling fins. According to the invention, a metallic cooling fin for a heat exchanger, of the type comprising a central part having at least one set of inclined fixed lamellae of selected form, the lamellae being spaced apart by apertures of selected dimensions such as to allow passage of a fluid between the lamellae, is characterised in that the lamellae in said set are distributed in at least two groups each consisting of at least one lamella, the respective inclinations of which vary as between one group and another, the groups having an order number which increases with the inclination of their respective lamellae.
The inclination in the lamellae of one group will thus be steeper as the order number assigned to the group is higher. Thus the lamellae in a first group will have a first inclination, the value of which is lower than that of a second group, which will itself be lower than that of any third group. To the extent that the fins have lamellae of at least two different inclinations, energy losses are considerably reduced.
Preferably, two adjacent lamellae, with different inclinations, constitute part of two groups of lamellae, the order numbers of which follow each other or precede each other. For example, a lamella of the first group will be followed by a lamella of the second group, the inclination of which is greater. Thus changes in direction of the fluid flow vary progressively, and therefore more gently, and this enables separation of the boundary layer to be limited, thus giving an increased working indirect heat transfer surface area.
According to a preferred feature of the invention, the lamellae which are located respectively in the first rank of a set of lamellae (that is to say at the start of the set) and or in the last rank of the same set (that is to say at the end of the set), form part of the group of lamellae of the lowest order number referred to as the first group. This makes the changes in direction of the fluid flow even more gentle.
In a preferred embodiment of the invention, the cooling fin has at least one set consisting of at least two groups of lamellae comprising a first and a second group, having first and second inclinations respectively, the said set of lamellae having a median plane of symmetry. With this feature, such a set of lamellae comprises at least one first lamella of the first group, followed by at least one lamella of the second group, which itself may be followed by at least one final lamella of the first group.
In addition, some cooling fins in the prior art include, upstream of the lamella of the first rank, a fixed upstream auxiliary lamella which is spaced away from the lamella of the first rank by an aperture of a selected form. This auxiliary lamella is adapted to channel the fluid flow at the start of the set of lamellae. It generally has a length which is substantially equal to one half of the length of the lamellae in the set, so that it does not redirect the fluid by a sufficiently large amount.
In order to overcome this problem, and in accordance with another preferred feature of the invention, the cooling fin includes, upstream of the lamella of the first rank, a fixed upstream auxiliary lamella, the dimensions of which are substantially equal to or greater than the lamellae in the set, the said upstream auxiliary lamella being spaced away from the lamella of the first rank by an aperture of selected form. Thus, the free end of the upstream auxiliary lamella is located at a lower level than the respective levels of the lamellae in the set, and this leads to effective redirection of the fluid flow. The fluid is then at once well oriented, while, firstly, a good approach is obtained to the working edges of the lamellae in the series, and secondly, the probability of separation of the boundary layer from the wall is considerably reduced.
Some prior art cooling fins also have, downstream of the lamella of the last rank, a fixed downstream auxiliary lamella which is spaced away from the lamella of the last rank by an aperture of selected form. This downstream auxiliary lamella is adapted, like the upstream auxiliary lamella, to channel the fluid at the terminal or downstream end of the set. The length of this downstream auxiliary lamella is generally substantially equal to one half of the length of the lamellae in the set, which again results in insufficient redirection of the fluid flow.
In order to overcome this problem, and according to yet another preferred feature of the invention, the cooling fin includes, downstream of the lamella of the last rank in a set, a fixed downstream auxiliary lamella, the dimensions of which are substantially equal to or greater than those of the lamellae in the said set, the downstream auxiliary lamella being spaced away from the said lamella of the last rank by an aperture of selected form.
Where the cooling fin has at least two sets of lamellae in succession (comprising an upstream set and a downstream set), these latter may be connected together through the upstream auxiliary lamella of one set and the downstream auxiliary lamella of the other set. The junction between two sets of lamellae is a factor in energy loss, and this feature improves the output of the heat exchanger.
According to yet another preferred feature of the invention, the auxiliary lamellae have an inclination which is smaller than or equal to that of the lamellae in the first group of the set.
In preferred embodiments with this feature the inclination of the auxiliary lamellae is smaller than that of the lamellae in the first set by an amount in the approximate range 1xc2x0 to 20xc2x0.
In some embodiments, that adjacent sets of lamellae have the same groups of lamellae. In that case, it is preferable that the groups having the same order number in two sets of adjacent lamellae have opposite orientations. This enables the fluid to be divided into layers, each of which penetrate between two lamellae of the upstream set, and leaves between the two corresponding lamellae of the downstream set, which are arranged symmetrically with respect to a median plane of symmetry.
In other embodiments, the adjacent sets of lamellae have different groups of lamellae.
Preferably, the inclinations of the lamellae are in the approximate range from 15xc2x0 to 35xc2x0. Sharp inclinations, typically greater than 30xc2x0, will sometimes no longer have any disadvantages, because their detrimental influence on the fluid is compensated for, at least by the fact that lamellae of different inclinations are used.
According to still a further preferred feature of the invention, the inclination of the lamellae in the first group is smaller than that of the lamellae in the group having the highest order number, by a value in the approximate range 1xc2x0 to 20xc2x0.
The invention is most particularly applicable to cooling fins made of alluminium or an aluminium alloy, or copper.